Molecular drag pumping stages produce pumping action by momentum transfer from a fast-moving surface (moving at a speed comparable to the thermal speed of the molecules) directly to gas molecules. Generally, these pumping stages comprise a rotor and a stator cooperating with each other and defining one or more pumping channels therebetween. Collisions of gas molecules in each pumping channel with the rotor rotating at a very high speed cause gas in the channel to be pumped from the inlet to the outlet of the channel itself.
The international patent application WO 2010/074965 in the name of the same Applicant of the present application discloses a molecular drag pumping stage comprising spiral pumping channels. In general, such a pumping stage comprises a stator ring having one or more spiral channels at least on a first face thereof and cooperating with the surface of a rotor disc rotating at high speed, the surface of the rotor disc being smooth and arranged opposite to the first face of the stator ring.
With reference to FIG. 1, if spiral channels are provided on both opposite faces of the stator ring, a first centripetal pumping stage 101 and a second centrifugal spiral pumping stage 102 connected in series can be obtained.
In detail a stator body 110 is provided on both surfaces 110a, 110b with spiral channels 112a and 112b, separated by corresponding spiral ribs 114a and 114b, respectively. A first rotor disc 116a having smooth surfaces is located opposite to a first surface 110a of the stator ring 110 and cooperates therewith for forming a first centripetal pumping stage 101 while a second rotor disc 116b having smooth surfaces is located opposite to a second surface 110b of the stator ring 110 and cooperates therewith for forming a second centrifugal pumping stage 102: the gas, coming from an inlet 118 placed at the outer periphery of the first pumping stage 101 flows through the first pumping stage in centripetal direction (arrow CP), passes through the passage 120 and then flows through the second pumping stage 102 in centrifugal direction (arrow CF), successively exiting through an outlet 122 placed at the outer periphery of the second pumping stage 102.
The pumping stages shown in FIG. 1 can be used in a vacuum pump in combination with other pumping stages of the same kind or of a different kind connected in series thereto, so that the gas flows through the pumping stages in centripetal and centrifugal direction alternately. Namely, the inlet 118 can put a previous centrifugal spiral pumping stage in communication with the first pumping stage 101 and the outlet 122 can put the second pumping stage 102 in communication with a successive centripetal spiral pumping stage.
Such pumping stages can also be used in combination with pumping stages of different kind, for instance they can be provided downstream a set of turbomolecular pumping stages in a turbomolecular pump.
A vacuum pump comprising a plurality of spiral pumping stages connected in series is shown in FIG. 2.
The vacuum pump 200 comprises a pump housing 202 in which a pump inlet 204 and a pump outlet 206 are defined and in which a plurality of spiral pumping stages are arranged between the pump inlet and the pump outlet.
To this aim, a plurality of stator rings 208, 210, 212 integral with the pump housing and provided with spiral channels 208a, 208b, 210a, 210b, 212a, 212b on both faces are arranged in the pump housing alternate with a plurality of rotor discs 214, 216, 218, 220 preferably having smooth surfaces and being mounted on a common shaft 222 that is centrally arranged in the pump housing and driven in rotation at high speed.
Thus, a plurality of alternate centripetal and centrifugal spiral pumping stages connected in series are obtained, and gas coming from the pump inlet 204 is pumped therethrough, as shown by the arrows F in solid line.
The vacuum pump 200 shown in FIG. 2 farther comprises a side inlet or additional inlet 224 provided at the side surface of the pump housing 202, between the pump inlet 204 and the pump outlet 206, namely between the first stator ring 208 and the second stator ring 210.
Both the pumping stage defined by the bottom face of the first stator ring 208 and the pumping stage defined by the top face of the second stator ring 210 face the side inlet 224. However, it will be evident from the above disclosure that the pumping stage defined by the spiral channels 208b on the bottom face of the first stator ring 208 does not participate in pumping a gas coming from the side inlet 224, as the spiral channels 208b define a centrifugal pumping stage.
Therefore, only the pumping stage defined by the spiral channels 210a on the top face of the second stator ring 210, which is a centripetal pumping stage, participates in pumping such gas coming from the side inlet 224, as shown by the arrows F′ in broken line.
More precisely, the gas coming from the side inlet 224 is not equally pumped by all the pumping channels 210a on the top face of the second stator ring 210, but mainly by the channels that are in flow communication with the additional inlet; in other words, assuming to longitudinally split each pumping stage in two halves H1, H2, the gas is pumped mainly by the channels that are in the half H1 comprising the additional inlet 224. Such situation is summarized in FIGS. 3a-3d, showing transverse cross-sectional views of the pump 200 at different, successive pumping stages.
Namely:
                FIG. 3a shows the centripetal pumping stage defined by the spiral channels 208a provided on the top face of the first stator ring 208 in cooperation with the first rotor disc 214; in this pumping stage only the gas coming from the pump inlet 204 is pumped (arrows F);        FIG. 3b shows the centrifugal pumping stage defined by the spiral channels 208b provided on the bottom face of the first stator ring 208 in cooperation with the second rotor disc 216; in this pumping stage only the gas coming from the pump inlet 204 is pumped (arrows F);        FIG. 3c shows the centripetal pumping stage defined by the spiral channels 210a provided on the top face of the second stator ring 210 in cooperation with the second rotor disc 216; in the channels of this pumping stage that are in communication with the additional inlet 224 both the gas coming from the pump inlet 204 (arrows F) and the gas coining from the side inlet 224 (arrows F′) are pumped, while in the remaining channels mainly the gas coming from the pump inlet 204 is pumped (arrows F);        FIG. 3d shows the centrifugal pumping stage defined by the spiral channels 210b provided on the bottom face of the second stator ring 210 in cooperation with the third rotor disc 218; in this pumping stage both the gas coming from the pump inlet 204 (arrows F) and the gas coming from the side inlet 224 (arrows F″) are pumped.        
It is evident from the above that the spiral pumping stages according to prior art cannot provide for an optimized configuration in case of vacuum pumps having a side inlet.
More particularly, since the pumping speed that can be attained at this additional side inlet is given by the sum of the pumping speeds of each pumping channel through which the gas is pumped, the limited number of pumping channels participating in the pumping of gas coming from the side inlet strongly limits the attainable effective pumping speed.
This and other objects are achieved by a spiral pumping stage as claimed in the appended claims and by a vacuum pump incorporating such pumping stage.